25 Jan 2010,
died on Saturday at 85. His
obituary
in the NY Times includes this paragraph:

Working in three dimensions, Mr. Chermayeff designed the sidewalk
sculpture — an immense number 9 in red steel —
that marks the entrance to
9 West 57th Street in Manhattan.
The building, by
Gordon Bunshaft
of Skidmore Owings & Merrill,
is noted for its convex facade that glides down to street level.

Well. 9 W 57 is concave, not convex.
(You could argue
that any concave surface is convex when regarded from the other side.
But a façade is generally
apprehended from without, no?) Obligatory screenshot
here
in case the Times
fixes*
their page.

On
19 May 2011,
I wrote about metastability, defined by Wikipedia as
"the ability of a digital electronics system to persist for an unbounded time
in an unstable equilibrium or metastable state." Metastability rears its
intractable head when (for example) a system has to decide which
of two events happened first.
See also
Buridan's ass.

Bitcoin's blockchain system is subject to metastability.
Each Bitcoin miner that strikes paydirt adds a new block to the blockchain
and announces it to the network. Should two miners extend the chain at
about the same time, the distributed database may fail to reach consensus
as to who was first. As described in the original
Bitcoin paper,
the can is kicked down the road:

If two nodes broadcast different versions of the next block simultaneously,
some nodes may receive one or the other first. In that case,
they work on the first one they received,
but save the other branch in case it becomes longer.
The tie will be broken when the next proof-of-work
is found and one branch becomes longer;
the nodes that were working on the other
branch will then switch to the longer one.

That works unless two miners again succeed at about the same time.
In practice these disputes are resolved before too long
but there is no guarantee of how quickly resolution will come.

*It's fixed as of 11 Dec 2017
(tacitly, i.e. without a correction notice)